Wear resistant cast irons have heretofore been primarily of the carbidic type. Unfortunately the carbides are normally massive and impart brittleness to the cast iron which limits its usefulness. The presence of manganese in gray or white cast irons exaggerates the formation of carbides because manganese acts as a carbide former. When manganese is present in excess of 1% by weight, the iron is difficult to machine. Such carbidic cast irons can be heat treated, but the desirable combination of physical characteristics represented in graphitized irons such as high toughness (50-60 ft/lb), high yield strengths (about 100 ksi), machinability and high tensile strengths (about 130-140 ksi) can never be achieved, regardless of the heat treatment. Because manganese is recognized as an effective carbide former, it is limited to amounts of 0.5-1.0% by weight in conventional graphitic cast irons (see "Describing the Eutectoid Transformation and Hyper-Eutectic Ductile Cast Irons", M. J. Lalich and C. R. Loper, AFS Transactions, 1973, pp. 238-244). As a result, such graphitic irons will have essentially a pearlitic matrix with less than desirable wear resistance and are difficult to machine.
Graphitizing agents, such as silicon and/or aluminum, are essential to the making of nodular ductile iron or compacted graphite iron, and will tend to counteract carbide formations even with high manganese contents; but the silicon or aluminum is balanced against the manganese in such irons because the iron still relies on the presence of carbide structures throughout the matrix of the metal to obtain some degree of wear resistance (see U.S. Pat. No. 2,885,284).
The use of manganese in ductile cast irons along with hardenability agents or pearlite-delaying alloying ingredients such as nickel, molybdenum and copper have been investigated and there is no evidence of the discovery of any method by which surface hardenable characteristics may be obtained while retaining all of the conventional desirable characteristics of ductile iron in the core of the cast iron part (see "An Investigation of the Effects of Alloying Elements in White Cast Iron", by Frick and Lindsay, AFS Transactions, 1974, pp. 65-70; and "Isothermal Transformations of Cementite in Ductile Iron", by Datta and Engel, AFS Transactions, 1974, pp. 267-278). To obtain the combination of good toughness and strength in the core and good wear resistance at the surface, the processing technique must be revised in a novel manner.